The present invnetion broadly relates to a new and improved method of manufacturing nonmagnetic drilling string components.
In its more particular aspects, the present invention relates to a method of manufacturing nonmagnetic drilling string components, in particular heavy-duty drill-stems for exploratory bores, e.g. for crude oil and/or natural gas deposits, such as directional bores or the like.
During the implementation of exploratory or prospecting bores, especially directional bores, the position and direction of the bores are established by magnetic measurement. Although measurements employing gyroscopic compasses have become known, preference is still accorded to the inherently very accurate and, when working with suitable materials, interference-free magnetic field measurements. Since bores of this type extend to increasingly greater depths, an especially exact position determination is required. This means that the drilling string components, in particular heavy-duty drill-stems which are located in the immediate proximity of the measuring instrument, for example, a so-called Forster probe, may only exhibit the most minute degree of magnetic anomalies. Thus it is known, for example, that heavy-duty drill-stems which produce a maxium compass deviation greater than .+-.0.25.degree., do not satisfy the requirements, or do so only in rare cases.
In addition to the nonmagnetic characteristics, as already explicity explained hereinbefore, these drilling string components, particularly heavy-duty drill-stems must possess high mechanical strength for withstanding tensile as well as compressive stress depending upon whether the drill head is subjected ot corresponding pressure or is withdrawn from the bore hole. Furthermore, these drilling string components, particularly heavy-duty drill-stems are subject to high torsional stress because the rotary motion of the drill head is at least partially carried out via such drilling string components. Furthermore, the alloys for such drilling string components, particularly heavy-duty drill-stems, must be suitable for providing thread connections which must be releasable without "jamming" or "seizing" even after long periods of exposure to mechanical loads.
A further, very relevant criterion is the resistance to corrosion, especially stress corrosion cracking, since such drilling string components are often exposed to highly corrosive media such as, for example, multipercent sodium chloride solutions and/or magnesium chloride solutions as well as hydrogen sulphide and the like.
In addition to the criteria set forth above, it is a further substantial prerequisite for the economical deployment of such drilling string components that alloys are employed which can be used over a long period of time and which therefore are not subject to the criterion of composition changes due to raw material shortages or the like.
From, for example, Austrian Pat. No. 214,466, there is known the use of a nonmagnetic austenitic chromium manganese steel alloy for manufacturing nonmagnetic drilling string components containing, each in percent by weight:
up to 0.25 carbon; PA1 up to 1.0 silicon; PA1 12 to 25 manganese; PA1 10 to 20 chromium; PA1 up to 5 nickel; PA1 up to 1 molybdenum; PA1 0.05 to 0.5 nitrogen; PA1 carbon in a maximum of 0.15, preferably 0.08; PA1 silicon in a maximum of 1.0; PA1 manganese 11.0 to 25.0, preferably 12.0 to 20.0; PA1 chromium 10.0 to 20.0, preferably 11.0 to 16.0; PA1 molybdenum 0.1 to 1.0; preferably 0.2 to 0.8; PA1 nickel 0.1 to 6.0, preferably 1.0 to 3.0; PA1 nitrogen 0.05 to 0.5, preferably 0.1 to 0.35;
and the remainder being iron and the usual accompanying elements. Boron, niobium, tantalum and vanadium, among others, are listed as further optional or non-essential alloying elements. A particularly suitable non-magnetic steel of this type contains, each in percent by weight, carbon up to a maximum of 0.12, silicon up to a maximum of 0.60, manganese 17.00 to 19.00, chromium 11.50 to 13.00, nickel 1.50 to 2.00, molybdenum 0.40 to 0.60, nitrogen 0.10 to 0.15, the remainder being iron and the usual accompanying elements.
The non-magnetic drilling strings are manufactured from such alloy by subjecting blanks to a number of processing steps including, among others, cold-working at room temperature in order to improve upon the mechanical properties of such austenitic steel, particularly increase the yield strength to the desired level. The thus-obtained drilling string components are not magnetizable by the strength of the terrestrial magnetic field. However, in stronger magnetic fields which can exist, for example, in the manufacturing environment, a permanent remanence can appear. When subjecting the thus manufactured drilling string components to an especially accurate final manufacturing control such as the method described in European Pat. No. 0,014,195, granted Sept. 16, 1981, ferromagnetic inclusions, pockets or islands were detected in the drilling strings which, therefore, had to be rejected because they are unsuited for carrying out the initially mentioned drilling operations because the ferromagnetic inclusions, pockets or islands cause unacceptable interferences with the initially mentioned magnetic measurements.
In a method of manufacturing blank material suitable for oil drilling nonmagnetic stabilizers such as known, for example, from U.S. Pat. No. 4,472,207, an ingot is produced from an austenitic nonmagnetic steel having the basic composition of, each in percent, carbon 0.05 to 3.0, manganese 16 to 25, and chromium 13 to 18. The ingot is subjected to a number of processing steps, such as hot-working in the temperature range of 800.degree. C. to 1200.degree. C., solution heat treatment in the temperature range of 1000.degree. C. to 1150.degree. C., water cooling, machining, cold compressive working in a specifically structured die in the temperature range of room temperature to 350.degree. C., stress relief annealing in the temperature range of 200.degree. C. to 350.degree. C. and finish machining. The thus obtained stabilizer had a specific magnetic permeability in the range of 1.002 to below 1.01 which was considered as satisfying the nonmagnetic conditions requirements.